How do I backup my iPhone with iCloud?
Apple's iCloud servers will automatically back up your iOS device, assuming that iCloud Backup is enabled. To enable iCloud navigate to Settings -> iCloud -> Backup & Storage.
The backup runs automatically every day when your device is:
Connected to the Internet via Wi-Fi
Connected to your charger
Your device screen is locked
You can back up with iCloud manually if you are connected to a Wi-Fi network by selecting Settings -> iCloud -> Storage & Backup -> Back Up Now.